This invention relates to a lead of an ordinary pencil or mechanical pencil, and more particularly it relates to a lead comprising a colorant and a filler dispersed in a binder, wherein the filler comprises a fibrous material.
It is already well known that a fibrous material can be used as at least part of a filler or extender in a lead comprising a colorant and a filler dispersed in a binder. For instance, Japanese Patent Publication No. 48-18347 discloses the use of fibrous graphite, carbon or the like, and Japanese Patent Laid-Open Specifications No. 52-88419 and No. 56-163171 disclose the use of fibrous potassium titanate.
The aim of using a fibrous material as a filler resides in an improvement in bending strength. Namely, it is anticipated that if a fibrous material is contained in an oriented state in a rod-like form, its bending strength could be increased.
However, even if these fibrous materials are actually used as the filler for a pencil lead, the binding strength can be rarely improved contrary to expectation. In addition, a pencil having such a lead often will not write well. This is because the hardness of these fibrous materials is too high and, in an extreme case, it is so high that the surface of writing paper is injured.
A fibrous material which can give a relatively good result is potassium titanate. Potassium titanate has a relatively low hardness, i.e., a Mohs hardness of 4-5, so that it can write sufficiently in practice. Further, it can provide a lead having a bending strength more excellent than that of a product containing no potassium titanate. However, it has a drawback that, when exposed to the air for a long time, its bending strength decreases with the lapse of time by the influence of moisture. This phenomenon may be considered to arise from the concentration of the absorbed moisture at the interface between the potassium titanate and the binder in the lead, with consequent poor adhesion therebetween.